In electronic packaging, thermal management is an important aspect of packaging design and it considers the issues associated with degradation of a device at high temperatures. Thermal management is becoming more important with the continuous trends for compact and more highly integrated systems having smaller features and higher currents. Conduction is one of the methods of heat transfer from a package or device in which the heat is transferred through a solid medium. The selection of materials used for the device and the substrate influence the ability of the device to conduct the generated heat away.
To rapidly transfer heat, heat-sinks such as copper, aluminum, or copper and aluminum alloys can be used. Copper is a better electrical and thermal conductor than aluminum. However, copper oxidizes. The formation of oxide on copper reduces its thermal conductivity. In addition, long term storage of the device after backside electroplating with copper can reduce the plated copper thickness due to corrosion. Copper oxide also has weak adhesion to copper. If the copper oxide separates, it can expose the copper underneath the oxide to air and form an additional oxide layer. Applying a thin layer of tin on copper protects the copper surface from corrosion and allows long storage of the device prior to installation. However, there are disadvantages related to the use of tin. Immersion tin chemistry contains a thiourea which is carcinogenic. Tin plating can create whiskers that present contamination and shorting problems. Tin does not work well with multiple reflow/assembly processes. Tin can aggressively attack solder masks. Tin forms an intermetallic with copper over time that results in the loss of pure tin, which results in degradation of solder performance. The first reflow exposure will dramatically reduce the pure tin thickness and the resulting deposit stress could result in tin whiskers. This is a naturally occurring characteristic of tin in direct contact with copper.